In these experiments an overhead propeller stirrer was used at a speed of 180 rpm. The experiments were carried out in a water bath at 25C. Experiments were carried out in triplicate to ensure reproducibility.
To determine the saturation degree necessary for precipitation, calcium and phosphate were introduced stepwise. After each addition of calcium and phosphate solutions, 30 minutes was given as an induction period before the next addition. Two samples were taken at each increment: sample 1 was taken 1 minute after addition and sample 2 was taken immediately before the next addition to check stability of the calcium and phosphate concentrations during this induction period. A continuous drop in pH indicated precipitation. In order to maintain constant experimental conditions during precipitation, and to produce enough precipitate for identification, a constant composition method was used (20). The principle of the method is that at the critical supersaturation, where nucleation begins, the drop in pH triggers the addition of calcium, phosphate and a pH adjusting solution (base). This maintains calcium, phosphate and pH at a constant level and allows the precipitate to grow. The experimental set-up is given schematically in figure 1.
Figure 1.Schematic experimental set-up for precipitation under constant composition conditions.